This invention relates to a method of tin coated steel sheet having excellent formability in the production of seamless steel containers by drawing and ironing operation.
Many processes are presently being utilized for forming seamless containrs from flat blanks. One of these procedures involves first drawing the blank into cup form by forcing the blank through a drawing die by means of a punch mounted upon a press. After drawing, the cup is passed through one or more ironing dies whose inside diameters are progressively smaller than the outside diameters of the cup.
As described in U.S. Pat. No. 3,360,157, "ironing" may be defined as thinning the walls of a deep-drawn article by reducing the clealance between punch and die.
In the ironing operation the side wall of the cup is elongated by reducing its thickness without reducing the inside diameter of the cup.
It is generally accomplished by forcing the cup and the punch through an ironing die whose diamerer is slightly less than the outer diameter of the cup and producing a longer seamless cup-shaped container with thinner side wall.
While the drawing and ironing process has been used in the manufacturing of seamless aluminum containers, tin coated steel sheet has also come into use as the material for seamless container, because the presence of surface tin coating enables a greater extent of deformation to steel sheet in the drawing and ironing operation.
A commercial tin coated steel sheet is made by continuous electrolytic plating of tin onto the steel, then the tin coating is normally melted and flow-brightened subsequently.
This commercial flow-melted tin coated steel sheet is found to need slightly greater force in the ironing operation than the non-flow-brightened tin coated steel sheet. However, the force needed is still much lower than the case of not-tinned plain steel.
As described in U.S. Pat. No. 3,360,157, the matte tin coated steel sheet that is produced without flow-brightened is more suitable to produce seamless containers by drawing and ironing, so that the matte tin coated steel sheet is used for this purpose.
However, the matte tin coated steel sheet has still room for improvement to accomplish the drawing and ironing easily without any fracture of the steel.
On the basis of such knowledge as described above, this invention provides a method for obtaining a tin coated steel sheet having excellent formability in the production of the seamless steel containers by forming an intermediate layer between steel and tin coatings.